Ch7. Chromosome Mutation Variation in Number and Arrangement. Although most members of diploid species normally contain precisely two haploid chromosome sets, many known cases vary from this pattern.
Ch7. Chromosome MutationVariation in Number and Arrangement
Although most members of diploid species normally contain precisely two haploid chromosome sets, many known cases vary from this pattern.
Modifications include a change in the total number of chromosomes, and changes of chromosomal structures, the deletion or duplication of genes or segments of a chromosome, and rearrangements of the genetic material either within or among chromosomes.
A change in Genetic material---mutation
Euploidy(): monoploidy and polyploidy
polyploidy :Autopolyploidy() Allopolyploids()
Aneuploidy(): loss 2n- 1 or more and add 2n+1 or more of the whole set of chromosomes
In humans, autosomal monosomy has not been reported beyond birth. There are, however, examples of survivors where only part of one chromosome is lost. These cases are sometimes referred to as segmental deletions.
One such case was first reported by Jerome LeJeune in 1963 when he described the clinical symptoms of the cri-du-chat (cry of the cat) syndrome. This syndrome is associated with the loss of part of the short arm of chromosome 5 (Figure 7-2). of
Thus, the genetic constitution may be designated as 46,5p-, meaning that such an individual has all 46 chromosomes but that some
A1A2a 3 (A1A2 a3 A1a3 A2 ) X 2 (a2A1 A2 ) x 2
P a/a x +/+/+
F1 a/+/+ a/a
2/6 a/+ 2/6 + 1/6 +/+ 1/6 a
eyeless female (ey ey) and a haplo-IV normal eye male (ey+).
PATAU syndrome = trisomy 13
- facial defects, polydactyly, heart defects, die within a few months of birth
EDWARDS syndrome = trisomy 18
- small + muliple defects, usually die in first year of life
DOWN syndrome = trisomy 21
The karyotype and phenotypic depiction of an infantwith Patau syndrome, where three members of the D-group chromosome 13 are present, creating the 47,13+ condition.
FIGURE 74 Incidence of Down syndrome births contrasted with maternal age.
FIGURE 7-6 Contrasting chromosome origins of an autopolyploid versus an allopolyploid karyotype.
FIGURE 7-7 The potential involvement of colchicine() in doubling the chromosome number, as occurs during the production of an autotetraploid. Two pairs of homologous chromosomes are followed. While each chromosome has replicated its DNA earlier during interphase, the chromosomes do not appear as double structures until late prophase. When anaphase fails to occur normally, the chromosome number doubles if the cell reenters interphase.
FIGURE 7-9 The origin and propagation of an amphidiploid().
Species I contains genome A consisting of three distinct chromosomes, a,, a2, and a3.
Species 2 contains genome 6 consisting of two distinct chromosomes, b1 and b2.
Following fertilization between members of the two species and chromosome doubling, a fertile amphidiploid containing two complete diploid genomes (AABB) is formed.
Genotype Gamete Ratio Phenotypes
AAAa 1AA1Aa all A
AAaa 1AA4Aa1aa 35A1a
Aaaa 1Aa1aa 3A1a
T tauschii 14
T searsii 14
T monocum 14
T dicocum 28
T timopheari 28
T zhukovskyi 42
T aestivum 42
Wild potato (2n=24)Cultivated Potato (2n=48)
Wild Cotton (2n=26)Cultivated Cotton (2n=52)
Dahlia (2n=32)Garden Dahlia (2n=64)
Wild Tobacco (2n=24)Cultivated Tobacco (2n=48)
For some plant species a series of successive ploidy levels are seen.
The New World cotton species Gossypium hirsutum has a 2n chromosome number of 52. The Old World species G. thurberiand G. herbaceum each have a 2n number of 26. Hybrids between these species show the following chromosome pairing arrangements at meiosis:
The second general class of chromosome aberrations includes structural changes that delete, add, or rearrange substantial portions of one or more chromosomes
Vitality by DELETIONS
Cell or chromosome behavior DELETIONS
Genetic effect by DELETIONS
DELETIONS deletions & human disease
FIGURE 7-11 The origin of duplicated and deficient regions of chromosomes as a result of unequal crossing over. The tetrad on the left is mispaired during synapsis. A single crossover between chromatids 2 and 3 results in deficient and duplicated chromosomal regions (see chromosomes 2 and 3, respectively, on the right).The two chromosomes uninvolved in the crossover event remain normal in their gene sequence and content
segments on homologue
The Bar Mutation in Drosophila
Gene Redundancy and AmplificationRibosomal RNA Genes
Although many gene products are not needed in every cell of an organism, other gene products are known to be essential components of all cells. For example, ribosomal RNA mustbe present in abundance in order to support protein synthesis. The more metabolically active a cell is, the higher is the demand for this molecule. We might hypothesize that a single copy of the gene encoding rRNA is inadequate in many cells. Studies using the technique of molecular hybridization, which allows the determination of the percentage of the genome coding for specific RNA sequences, show that our hypothesis is correct! Indeed, multiple copies of genes code for rRNA. Such DNA is called rDNA, and the general phenomenon is called gene redundancy.
The Role of Gene Duplication in Evolution
A B C D E F G H
A B F E D C G H
Consequences Inversions of Inversions During Gamete
D / +
++/++ D+/++ D+/+G l ++/+Gl
D+/D+ D+/+Gl +Gl/+Gl
Cy + Cy +
+ A + A
Cy + Cy + + A
Cy + + A + A
variegated type of position effect
Activator-Dis sociation system
Translocations in Humans:Familial Down Syndrome
7.11 Fragile Sites in Humans
Fragile X Syndrome (Martin-Bell Syndrome
Fragile Sites and Cancer